1. Field of the Invention
The present invention relates generally to the process of manufacturing a capillary tube for use in a refrigeration system, and more specifically, to the process of shaping a length of tube having at least one axially disposed groove longitudinally to a preselected axial configuration, and then squeezing the tubing in two stages, and at different speeds while passing a medium therethrough under pressure until substantially all of the medium passes through the groove, reaches a preselected pressure.
2. Description of the Prior Art
In providing capillary tubes that meet the proper restriction requirements of a particular refrigeration system, it has been customary practice to purchase readily available tubing from standard stock having the smallest inside or bore diameter, and then cutting it to the length that would provide the proper restriction needed for a particular application.
While this practice does provide a capillary tube having effective flow characteristics, the proper flow restriction, however, is determined by the length of tubing and not by a predetermined passageway or inside diameter dimension. To accommodate the relatively long capillary provided in this manner into the available cabinet space, it is often necessary to coil or shape it. This coiling and handling of the tube presents some hazards in that the relatively small diameter passageway of the tubing may be pinched and closed off during the handling and shaping.
U.S. Pat. No. 2,225,513--Sommers discloses a method of forming restrictors wherein a relatively straight length of tubing is provided with longitudinally formed grooves that deform it cross-sectionally to initially reduce its passageway. The deformed tube is then twisted while being maintained in a straight longitudinal configuration. During the twisting operation a gas under constant pressure is passed through the tube with the twisting operation being terminated when the gas reaches a desired pressure. While the Sommers patent does provide a capillary having a predetermined length, it is, however, formed in a straight line and in some instances would require its shaping to fit its free ends into engagement with the appropriate tube members to complete its connection into the refrigeration system. In fact, unless the open ends of the refrigeration system to which the capillary is to be connected are spaced to receive the straight length of capillary formed by the Sommers patent, it must when completed be shaped either to fit into the available spaces or to align its ends with the proper refrigeration tube connections.
In forming standard tubing of preselected lengths into capillaries having a precise restriction, the time involved in forming them is extremely important when related to mass production systems. When tubing is formed in a slow continuous squeezing or forming operation until the proper restriction is achieved it has been found uneconomical in that mass production schedules and volume commitments are difficult to maintain.
In forming capillaries from standard tubing, that is, tubing available as a stock item, the use of a single high speed continuous squeezing or forming operation has been found to be impractical in that the reaction time between the termination of a squeezing operation after the proper restriction is sensed or arrived at, is sufficient in some instances to cause an overshoot. This overshoot caused by the reaction time between the sensing of the proper restriction and the ability to stop a high speed machine results in an overly restricted tube or may in fact cause the passageway to be completely shut off, in which case it is then necessary as disclosed in U.S. Pat. No. 1,392,658--Roesch, to provide means and the additional step of reopening the closed passageway. To reopen the passageway and afford the precise slow characteristics required would present many of the same problems faced in prior art attempts and would not be economically feasible in meeting the mass production requirement.
In addition, it has been the experience in squeezing tubes with high speed equipment that the squeezed tube has a tendency to spring back. In effect, when a high speed operation or machine is brought to an abrupt stop at a precise preselected dimension the inability to accurately determine what the spring back qualities of the tube will be results in inaccurate restriction configuration.
Another method of manufacturing capillary tubes is disclosed in the pending patent application Ser. No. 577,710--Pohl et al filed May 15, 1975 and assigned to General Electric Company, the assignee of the present invention. The Pohl application, of which the present invention is an improvement, discloses a method wherein a length of tubing is pre-shaped and a selected portion is squeezed in two steps until medium passing through the flattened squeezed portion reaches a predetermined pressure. The restriction in the tube member at the time the proper pressure is attained in effect is an extremely narrow slit which may be subject to clogging by foregoing objects that may be present in the refrigeration system.